California's cultivation of marijuana has created an unprecedented phenomenon: southbound smuggling of the drug across the U.S.-Mexico border.

Mexico's demand for potent California strains is on the rise as Mexican drug cartels have mostly failed to make a competitive homegrown product.

"If you're in Mexico, and you want the best marijuana out there, there's only one place to get it," said Matthew Shapiro, a San Diego-based attorney who specializes in marijuana. "There's no such thing as high-quality Mexican weed."

California's initiative to legalize marijuana for recreational use could further boost southbound smuggling, experts on both sides of the border told KPBS. It would make it easier for Tijuana residents with visas or dual citizenship to access California's more potent strains -- and bring it back to Mexico.

It's illegal to move marijuana from the U.S. to Mexico, just as it's illegal to move the drug from Mexico into the U.S. But it's easier to smuggle southbound. At the San Ysidro Port of Entry, drivers can cross the border into Tijuana without ever stopping to speak with a Mexican official.

Tijuana's public drug rehabilitation clinic,Centro de Integración Juveníl, has seen an increase in patients with self-described addictions to California marijuana, largely Mexicans with visas or dual citizenship who have access to San Diego dispensaries or friends with medical cards.

"They can get it easily in the U.S., and they bring it back with them in small quantities -- generally for their own consumption," clinic director Raul Palacios said. "I don't doubt that they share it with their friends, but not on a large scale."

Palacios said his patients prefer California's medical marijuana because it has a higher concentration of THC, the plant's main psychoactive ingredient. The Mexican marijuana sold by street dealers in Tijuana has a roughly two percent concentration of THC, he said, while the California-grown product's concentration can exceed 30 percent.

But the potency of the California product can be perilous for new users, Palacios said. The stronger marijuana makes patients more prone to symptoms of psychosis. Some of the recovering California marijuana users atCentro de Integración Juveníl, experienced hallucinations and paranoid delusions.

"So its psychoactive capacity, or psychoactive effect, is more damaging," Palacios said of California marijuana.

In Colorado, marijuana-related emergency room visits have spiked since it was legalized in 2012. Notably, the increase is most pronounced among out-of-state visitors who aren't used to the state's high-THC marijuana. Those users suffer psychosis, car accidents and other problems more often than state residents.

Similarly, Tijuana residents, accustomed to smoking weaker Mexican marijuana, may have a lower tolerance for the high-THC marijuana from California.

KPBS conducted interviews with some of the Tijuana clinic's patients on condition that they be identified only by their first names. In Mexico, there is a much stronger stigma associated with smoking marijuana than in the U.S.

One patient, Antonio, said he is able to purchase marijuana in California because he has a visa. He prefers it over Mexican weed, even though it's more expensive.

"The quality is much better over there," he said.

His mother, Yolanda, said California's medical marijuana made him hallucinate. She said he was convinced people were filming him and following him.

"He doesn't want to talk to me. He writes to me. He writes down his thoughts, because if we talk, he thinks it's going to be published online," she explained.

When it comes to marijuana, Mexico is a much more conservative country than the U.S. Less than a third of people in Mexico are in favor of legalizing marijuana, compared with more than 60 percent in the U.S.

Earlier this year, Mexico's President Enrique Peña Nieto said he was open to the idea of legalizing marijuana for medical use. The government is also considering decriminalizing possession of small quantities of marijuana.

"Mexico is going to try to imitate the U.S.," said Rodrigo, another patient, who has dual citizenship.

He said he started smoking marijuana after it was legalized for medical use in California. He thought it couldn't be harmful if a U.S. state was recognizing its health benefits.

But the weed, which he got from friends in San Diego, made him hallucinate, he said.

"I thought I was being followed," he said.

That's why he sought treatment at the clinic.

Rodrigo said he always preferred California's marijuana over the product available in Tijuana.

"It's more powerful, it gets you higher," he said.

He said if marijuana is legalized for recreational use in California, the temptation -- and the risk -- for Tijuana residents like him is going to rise.